Atmospheric Stability & Cloud Development

Transcription

1 Atmospheric Stability & Cloud Development Stable situations a small change is resisted and the system returns to its previous state Neutral situations a small change is neither resisted nor enlarged Unstable situations Neutral a small change initiates a bigger change, and hence a bigger still, and hence.. Stable Unstable 1

4 Rising & Falling The atmosphere is a fluid whose density and pressure generally decreases with height The fluid has irregularities in it An air parcel will rise if it is less dense than Air parcel its surroundings It then rises into less dense air, but the density inside the packet also falls If it stops rising, it is stable; if it keeps rising, it is unstable 4

5 Density Depends on Temperature Pressure P, volume V, and temperature T, of a parcel of gas are linked by a relationship for an ideal gas (the simplest model of a gas) PV = nrt R is 'the gas constant', per mole, and n the no. of moles If two parcels of gas are at the same pressure, then the warmer gas has the lesser density this follows because the gas law above can be written [p 212/220/194] P density T 5

6 Adiabatic Changes in a Parcel of Gas When a parcel of gas expands on rising, how much the volume changes depends on how the expansion takes place adiabatic:- no heat input or output: PV γ = constant P = P 0 e -h/h γ is about 1.4 for air This relationship, along with the gas law and the pressure variation with height determines how temperature changes with height (next slide) 6

7 Adiabatic Lapse Rates As a parcel of air expands without heat input from its surroundings, its pressure and temperature drop and its volume increases Meteorologists concentrate on the temperature drop the temperature drop with changing height is called the lapse rate for adiabatic change of dry air, the dry adiabatic lapse rate is 10 C per 1000 m [p. 160/167/140] the moist adiabatic lapse rate is less (~6 C per 1000 m) why? Adiabatic expansion 7

8 In summary 8

9 Atmospheric Temperature Profile The profile of temperature change with height in the atmosphere depends on the history of the air this profile can be measured by a balloon borne thermometer the result is called the environmental lapse rate lapse rate is measured in C per 1000 m Height m Environmental lapse rate e.g. 5 C per 1000 m in the diagram 0 10 C 20 9

13 Unstable Conditions Formed by warming of lower air layers by radiation during the day by air moving over warm ground by influx of warm air (warm advection) enhanced effect if moist air near ground and dry air aloft is lifted (convective instability) 13

16 Looping In unstable conditions, the plume rises and falls as light winds take it away. If this happens without too much dispersion of pollutants, the plume follows a looping path 16

17 Coning In neutral conditions, the environmental lapse rate equals the dry adiabatic lapse rate and the plume spreads up and down about equally without turbulence 17

18 Lofting Stable air near the ground and unstable air aloft gives no downward mixing and the best conditions for sending plumes upwards [fig. 17.3/ chpt. 18 fig. 4, p. 459/220/506] with a good exit velocity and high temperature of pollutants the plume will rise faster than shown 18

21 Cumulus (Cu) Formation The cumulus cloud is part of a convective cell local heating induces instability in the air (fig. 7.14/6.16) the cloud base is at the height where air reaches its dew point Harbour entrance, Aberdeen JSR 21

22 Dew Point Revisited For every rise of 1000 m in height, pressure falls by 10% when water vapour pressure falls by 10%, the dew point falls by 2 C Hence for every 1000 m rise in height, dew point falls by 2 C [p. 172/177/153] 22

25 Cumulus Clouds Fig 7.16/6.18 [previous slide] is a graphic summary rising air in the cloud remains saturated the cloud height is controlled by a stable layer on top (which may be the tropopause) outside the cloud, air sinks, warms and creates a cloud-free space as a layer of air rises and stretches, its lapse rate tends to increase, making it more unstable and liable to rise further. See the earlier slide on this topic JSR 25

26 Alto- and Cirrocumulus Formation A uniform layer of cloud gets heated from below by the Earth and loses heat from the top to space (p176/182/156) This increases the lapse rate across the cloud and can make the air unstable Convection cells result convective instability further increases the vertical extent as lifting begins, since drier air at the top cools faster than moist air at the bottom JSR 26

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